Many ages ago not long after the first caveman whittled a piece of silicon into a transistor, they learned how to make a bunch of transistors on the same hunk of silicon. This led to all kinds of things that we consider as always having been around, like the 741 opamp, 7805 regulator, and 2N2222 transistor. Yes, these things actually had beginnings. They didn't spring from the big bang fully formed as the myths tell us today.
Several groups of advanced cave men came up with lots of new and clever ways to whittle their silicon. A group calling themselves "National" brought forth the 741 opamp and the 7805 regulator, and the "Signetics" gang originated this 555 timer thing. It was pretty cool for its day. It is basically two comparators with fixed thresholds of 1/3 and 2/3 of the supply, and a flip-flop and some control circuitry to make other pins do things depending on what the comparators were seeing. You could wire up one of these things to self-oscillate, or produce a pulse of a certain width, delay a pulse, etc.
Eventually of course, we moved out of caves and got better and better at whittling silicon. Today we have many ways to do the things the ancient 555 timer did, but for less money, less board space, and a lot less power. The 555 timer deserves its recognition in the museum of neolithic inventions, but that is its place today. Some of the original cavemen are still around and still use 555 timers because that's what they are comfortable with, but those that have kept up with electronics since the ancient cave days leave the stone knives, bearskins, and 555 timers in the museum.
So today we appreciate the 555 timer for its place in the past, but it is not something for new designs except to honor history, for the few cavemen that are still with us, or for those that practise the ancient rituals for their own sake.